1. Field of the Invention
The present invention relates to an organic electroluminescent display (OELD) device, and more particularly, to an OELD device including a hole injection layer having an improved properties and method for fabricating the same.
2. Discussion of the Related Art
In general, organic electroluminescent display (OELD) devices emit light by injecting electrons from a cathode and holes from an anode into an emission material layer, combining the electrons with the holes, generating excitons, and transforming the excitons of an excited state to a ground state. Unlike liquid crystal display (LCD) devices, OELD devices do not require an additional light source and therefore have the advantages of slimness and lightweight.
Because OELD devices have excellent characteristics, such as low power consumption, high luminance, fast response time, lightweight and so on, OELD devices can be applied to various electronic products, such as mobile phones, car navigation system (CNS), PDAs, camcorders, palm PCs, and so on. Moreover, due to their simple fabricating process, the fabrication costs of OELD devices are low when compared with those of LCD devices.
The OELD devices are divided into a passive matrix type and an active matrix type according to the driving method thereof. The passive matrix type OELD devices have a simple structure and a simple fabricating process. However, the passive matrix type OELD devices have disadvantages of high power consumption and low quality images. Moreover, the more lines are disposed, the less aperture ratio does the passive matrix type OELD device have. On the other hand, the active matrix type OELD devices have advantages of high emission efficiency and high quality images.
FIG. 1 is a cross-sectional view illustrating an active matrix type OELD device according to the related art.
Referring to FIG. 1, the OELD device 10 includes first and second substrates 12 and 28 facing each other. The first substrate 12 is transparent and flexible. The first substrate 12 has an array element 14 including a plurality of thin film transistors (TFTs) T and an organic electroluminescent diode E including a first electrode 16, an organic luminescent layer 18 and a second electrode 20. The organic luminescent layer 18 in each pixel region P includes one of red, green and blue color materials.
The second substrate 28 includes a moisture absorbent 22 of a powder type. The moisture absorbent 22 removes moisture inside the OELD device 10. The moisture absorbent 22 is in a concave portion of the second substrate 28 and is sealed by a taping 25. The first and second substrates 12 and 28 are attached to each other with a seal pattern 26.
In the OELD device 10, because the first electrode 16 is formed of a transparent material, the light emitted from the organic luminescent layer 18 travels toward the first substrate 12. Accordingly, it is referred to as a bottom emission type OELD device.
FIG. 2 is a circuit diagram of an OELD device according to the related art.
Referring to FIG. 2, gate and data lines 40 and 50 are formed on a substrate 32. The gate and data lines 40 and 50 cross each other and a switching element Ts is formed near the crossing portion of the gate and data lines 40 and 50. The switching element Ts includes a gate electrode 42, a source electrode 52 and a drain electrode 56. The gate electrode 42 is connected to the gate line 40. The source electrode 52 separated from the drain electrode 56 is connected to the data line 50.
A driving element Td is electrically connected to the switching element Ts. The driving element Td of a p-type TFT includes a gate electrode 44, a source electrode 62 and a drain electrode 66. The gate electrode 44 of the driving element Td is connected to the switching element Ts. A storage capacitor Cst is formed between the source and gate electrodes 62 and 44 of the driving element Td. The drain electrode 66 of the driving element Td is connected to the first electrode 16 (of FIG. 1) of the organic electroluminescent diode E. The source electrode 62 of the driving element Td is connected to a power line 47.
When a gate signal from the gate line 40 is supplied to the gate electrode 42 of the switching element Ts, a data signal from the data line 50 is supplied to the gate electrode 44 of the driving element Td through the switching element Ts. Then, the organic electroluminescent diode E is driven by the driving element Td such that the organic electroluminescent diode E emits light. Because the storage capacitor Cst maintains a voltage level of the gate electrode 44 of the driving element Td, even if the switching element Ts is turned off, the organic electroluminescent diode E can continuously emit light for a predetermined period of time.
FIG. 3 is a plan view illustrating a pixel region in an array substrate of an active matrix type OELD device according to the related art and FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 3.
Referring to FIGS. 3 and 4, the active matrix type OELD device includes a switching element Ts, a driving element Td and a storage capacitor Cst on a substrate 32. Each pixel P of the OELD device may include more than one pair of the switching element Ts and the driving element Td. The substrate 32 is formed of a transparent and insulating material, such as glass and plastic.
A gate line 40 and a data line 50 are formed on the substrate 32 with an insulating layer interposed therebetween. A power line 47 is formed along the data line 50. A pixel region P is defined by the crossing between the gate and data lines 40 and 50.
The switching element Ts may be a thin film transistor including a first active layer 34, a gate insulating layer 39, a gate electrode 42, a first interlayer insulating layer 41, a second interlayer insulating layer 48, source and drain electrodes 52 and 56. The gate electrode 42 of the switching element Ts corresponds to the first active layer 34 and is connected to the gate line 40. The first interlayer insulating layer 41 is formed on the gate electrode 42 and has a first contact hole 51 and a second contact hole 54. The source electrode 52 of the switching element Ts is connected to the data line 50 and is spaced apart from the drain electrode 56 of the switching element Ts. The source and drain electrodes 52 and 56 are insulated from each other due to the second interlayer insulating layer 48. The drain electrode 56 of the switching element Ts is connected to a gate electrode 44 of the driving element Td through a third contact hole 55.
The driving element Td may be also a thin film transistor including a second active layer 36, a gate insulating layer 39, a gate electrode 44, a first interlayer insulating layer 41, a second interlayer insulating layer 48, source and drain electrodes 62 and 66. The gate electrode 44 of the driving element Td corresponds to the second active layer 36 and is connected to the drain electrode 56 of the switching element Ts through the third contact hole 55. The first interlayer insulating layer 41 is also formed on the gate electrode 44 of the switching element Td and has a fourth contact hole 61 and a fifth contact hole 65. The source electrode 62 of the driving element Td is connected to the power line 47 through a sixth contact hole 49 and is spaced apart from the drain electrode 66 of the driving element Td. The source and drain electrodes 62 and 66 of the driving element Td are insulated from each other due to the second interlayer insulating layer 48.
A first passivation layer 68 is formed on the switching and driving elements Ts and Td and exposes a portion of the driving electrode 66 of the driving element Td. An organic electroluminescent diode E having a first electrode 70, an organic luminescent layer 74 and a second electrode 76 is formed over the substrate including the switching and driving elements Ts and Td. The first electrode 70 is connected to the drain electrode 66 of the driving element Td, and the organic luminescent layer 74 is interposed between the first and second electrodes 70 and 76. In addition, a second passivation layer 72 is formed on the first electrode 70 to insulate from the second electrode 76.
Moreover, an impurity-doped silicon layer 38 is formed on the substrate 32 and under the power line 47. The impurity-doped silicon layer 38 and the power line 47 respectively function as first and second capacitor electrodes, and the first interlayer insulating layer 41 therebetween functions as a dielectric material. The first capacitor electrode, the second capacitor electrode, and the dielectric material constitute a storage capacitor Cst.
FIG. 5 is a schematic cross-sectional view of an organic electroluminescent diode of a conventional OELD device.
Referring to FIG. 5, the OELD device includes the organic electroluminescent diode E over the substrate 32. The organic electroluminescent diode E includes the first electrode 70, the organic luminescent layer 74 and the second electrode 76. The first and second electrodes 70 and 76 may be referred to as an anode and a cathode, respectively. When voltages are applied to the first and second electrodes 70 and 76, the first and second electrodes 70 and 76 respectively output holes and electrons. The organic luminescent layer 74 includes a hole injection layer (HIL) 74a, a hole transporting layer (HTL) 74b, an emission material layer 74c including a red emission material layer EML(R), a green emission material layer EML(G) and a blue emission material layer EML(B), an electron transporting layer (ETL) 74d and an electron injection layer (EIL) 74e. Because an organic material has very different mobilities with respect to the hole and the electron, the organic electroluminescent diode E includes the hole transporting layer 74a and the electron transporting layer 74d to effectively transport the holes and electron from the first and second electrodes 70 and 76 into the emission material layer 74c. It is possible to obtain a balance between the holes and the electrons such that emission efficiency is improved. The organic electroluminescent diode E includes the hole injection layer 74a between the first electrode 70 and the hole transporting layer 74b and the electron injection layer 74e between the second electrode 76 and the electron transporting layer 74d. The hole injection layer 74a decreases a barrier energy for injecting the hole into the emission material layer 74c, and the electron injection layer 74e decreases a barrier energy for injecting the electron into the emission material layer 74c. As a result, there are advantages that emission efficiency is improved and driving voltage is reduced. In generally, when the hole injection layer 74a is formed of a crystallizable material such as perylene, tri-phenylene, it is much effective to reduce the driving voltage. However, the crystallizable material has an un-uniform arrangement of grains. It causes deterioration in the organic electroluminescent diode E and decrease of lifetime of the OELD device.
FIG. 6 shows an arrangement of grains in a hole injection layer according to the related art, FIG. 7 is a picture showing a surface of hole injection layer according to the related art, and FIG. 8 is a picture showing an emission state of an organic electroluminescent diode according to the related art.
As shown in FIG. 6, a first electrode 70 of a transparent conductive material, such as indium-tin-oxide (ITO), and a hole injection layer 74a of a crystallizable material are laminated over a substrate 32. The transparent conductive material of the first electrode 70 may have one of amorphous phase and crystallized phase. The amorphous phase is changed into the crystallized phase by heating. FIG. 6 shows the first electrode 70 of the crystallized phase. Because the ITO material having the crystallized phase has a uniformly arranged grains A, the first electrode 70 has an even surface to have an excellent interface with the hole injection layer 74a. However, grains B of the hole injection layer 74a of the crystallizable material are un-uniformly arranged. Some grains protrude from the upper surface of the hole injection layer 74a. Accordingly, the hole injection layer has a uneven upper surface.
Referring to FIG. 7, the grains B has different sizes. Because light emitted from the organic luminescent layer 74c passes through the hole injection layer 74a, the protruding grains from the upper surface of the hole injection layer 74a cause defect sites D. When the OELD device having the above mentioned hole injection layer 74a is driven, currents are concentrated into the defect sites D to have brightness much than other sites. Namely, the defect site D becomes a bright site. Moreover, as the concentration of the currents is progressed, the defect sites D have brightness less than the other sites. Namely, the defect site D becomes a dark site. It causes a deterioration problem in the emission material layer 74c (of FIG. 6).
Referring to FIG. 8, the OELD device can not produce a uniform brightness due to the defect sites D (of FIG. 7) of the hole injection layer 74a (of FIG. 6). Accordingly, the OELD device does not display high quality images. Moreover, lifetime of the OELD device decreases because of the deterioration problem in the emission material layer 74c (of FIG. 6).